1. Field of the Invention
The present invention relates to switching power conversions, and more particularly to switching power conversions capable of turning off a secondary side switch when abnormal load variations or light load status is detected.
2. Description of the Related Art
In supplying the power for electronic equipments, switching power converters are widely adopted due to the advantages of high conversion efficiency and small component size they possess.
Taking the flyback AC-to-DC power adapter as an example, FIG. 1 shows an illustrating block diagram of a prior art flyback AC-to-DC power adapter including a secondary side synchronous rectification controller. As shown in FIG. 1, the architecture includes a NMOS transistor 101, a main transformer 102, a NMOS transistor 103, a capacitor 104 and a secondary side synchronous rectification controller 105.
In the architecture, the NMOS transistor 101 is used to control the power transformation through the main transformer 102 in response to a PWM signal VG1.
The main transformer 102 is used to transfer the input DC power source VIN to a DC output voltage VO.
The NMOS transistor 103 is coupled to the secondary side of the main transformer 102 to emulate a rectification diode to cut off the current path at the secondary side when the NMOS transistor 101 is on and release the magnetic flux to the capacitor 104 and a load (not shown in FIG. 1) when the NMOS transistor 101 is off under the control of a secondary side gating signal VG2 from the secondary side synchronous rectification controller 105.
The capacitor 104 is used for carrying the DC output voltage VO.
The secondary side synchronous rectification controller 105 is used to generate the secondary side gating signal VG2 to switch the NMOS transistor 103 according to a secondary side voltage signal VD which spikes high when the NMOS transistor 101 is turned on.
Through a periodic on-and-off switching of the NMOS transistor 101, which is driven by the PWM signal VG1 generated from a PWM controller (not shown in FIG. 1), and a corresponding switching of the NMOS transistor 103, the input power is transformed through the main transformer 102 to the output.
However, when the DC output voltage VO experiences steep load transitions, the turn-on time of the NMOS transistor 101 controlled by the PWM signal VG1 will exhibit large variations for a transient period, and the NMOS transistor 103 may thereby not be switched at the right instants during the transient period due to possible failure of the secondary side gating signal VG2 to follow the dramatic variations of the turn-on time of the NMOS transistor 101, which will pose a risk of damaging the adapter. In addition, when the adapter is under a light load, the power dissipated in driving the NMOS transistor 103 may be more than that dissipated in a rectification diode of an adapter without a secondary side synchronous rectification controller.
Therefore, there is a need to provide a solution capable of switching off the secondary side switch appropriately when the adapter experiences steep load transitions or light load condition, to prevent possible disasters and/or minimize the power consumption.